1. Field of the Invention
The present invention relates to antenna devices and wireless communication devices that include the antenna devices, and particularly relates to antenna devices and wireless communication devices preferably for use in HF band communication systems.
2. Description of the Related Art
RFID (Radio Frequency Identification) systems in which a reader/writer and an RFID tag are caused to carry out non-contact communication and information is transmitted between the reader/writer and the RFID tag are known. Both the reader/writer and the RFID tag have an antenna device for sending and receiving wireless signals. If the RFID system is an HF band RFID system that uses a 13.56 MHz band, for example, the antenna device in the RFID tag and the antenna device in the reader/writer couple primarily using an induction field, and then send and receive predetermined information.
RFID systems are recently being introduced into communication terminal devices such as cellular phones, and such communication terminal devices are sometimes used as reader/writers, RFID tags, and so on. Japanese Unexamined Patent Application Publication No. 2002-63557 discloses a ferrite chip antenna that is able to be used in such an RFID system.
With miniaturization, higher levels of integration, and so on in communication terminal devices, small electronic devices, and the like used in communication systems such as the aforementioned RFID system, there is demand for the miniaturization of antenna devices as well. However, the radiation capabilities of an antenna device generally decrease due to such miniaturization, which tends to narrow the required communication band width. It is therefore desirable for the characteristics of the antenna device itself to have wide band characteristics and be small, or have wide band characteristics relative to the small size.
Meanwhile, when an antenna device is incorporated into a limited space along with a circuit board, structural members, and so on, the antenna device is often near ground electrodes, metal members, and the like; being near such conductors alters the inductance of the antenna, causing a resonant frequency to deviate from an expected value (a desired value). It is thus desirable for the resonance characteristics of the antenna device to span a wide band so that the desired effect can be obtained even if the resonant frequency of the antenna device changes.
Furthermore, when an antenna device is used in both a reader/writer mode and a card emulation mode, the impedance of a power supply circuit in an RFIC connected to the antenna device will vary depending on the mode, even if the antenna device is in a stable surrounding environment. It is thus desirable for the resonance characteristics of the antenna device to have a wide band so that predetermined communication performance can be achieved even if the impedance of the power supply circuit changes.
Broadening the band based on the various aforementioned requirements is particularly important in narrow-band communication systems such as NFC (Near Field Communication) systems, where the band width ratio is several percent.